There exist demands for electronic devices flexible, light-weight, favorable in appearance and convenient in use in daily life without particular recognition of its presence that are aimed at use in ubiquitous information society where information is available anytime anywhere. However, silicon which is a typical conventional semiconductor material, cannot satisfy these demands sufficiently. Recently, organic semiconductor materials are attracting attention as materials satisfying such demands. In particular, organic semiconductor materials allowing film forming by wet process such as solution coating method have a possibility of production of large-area elements at lower temperature and lower cost (see, for example, Advanced Materials, 2004, vol. 16, p. 312-316, and Applied Physics Letters, 2006, vol. 88. p. 113511). Electronic devices having these properties that are also transparent and colorless, if produced, would be useful in establishing such a ubiquitous information society.
In image-displaying devices such as liquid crystal display and organic EL display and also in light-receiving devices such as optical sensor, if an opaque semiconductive circuit for example of conventional silicon is used as a selective driving circuit, the selective driving circuit and the an apertural area should be formed separately on different portions, leading to decrease numerical aperture. It would be possible to raise the numerical aperture significantly if a transparent and colorless thin film transistor (TFT) can be used as the selective driving circuit, because the transistor can be laminated with an image-displaying element or a light-receiving element. However, there is currently no such thin film transistor satisfying the requirements both in transparency and transistor characteristics at the same time.
For use as the selective driving circuit, the TFT may be a p-type transistor or n-type transistor. Among these, p-type organic semiconductor materials are often superior in properties, and generally the properties of p-type organic TFT are better, because its operation stability in air is higher.
Examples of the transparent organic thin film transistors containing the p-type organic semiconductor material in an active layer include those using pentacene (see, for example, Advanced Materials, 2004, vol. 16, p. 312-316) and those using vanadyl phthalocyanine (VOPc) (see, for example, Applied Physics Letters, 2006, vol. 88, p. 113511). Although these materials have light absorption in the visible range, they are used as substantially transparent thin film transistors, as the absorbance thereof was made smaller as they are used as a thin film. Although these organic thin film transistors are relatively transparent, there still exists continuing need for improvement in transparency and colorlessness.